In a common spring powered stapler a handle is linked to a rear end of a lever, and the front end of the lever is linked to a striker. Pressing the handle causes the lever to pivot about a lever fulcrum. According to one design the front end of the lever moves upward in an arcing motion so that the lever moves rearward as the lever front end approaches its upper limit. At a predetermined position of the lever the striker is disengaged from the lever. The striker then moves downward from the bias of a power spring to eject a staple from the stapler.
U.S. Pat. No. 5,988,478 (Marks) shows a lever and a power spring where each respective component has a separate and distinct fulcrum. U.S. Pat. No. 6,145,728 (Marks) shows a staple gun where a power spring and a lever share a common fulcrum. The lever is a “U” channel design and the power spring is an elongated flat spring. The shared fulcrum provides minimal net force on the fulcrum and thus internal forces since the lever and spring press with approximately equal and opposite forces on the fulcrum. Especially when plastic material is used it is desirable to limit internal forces in the stapler to minimize distortion of the housing. U.S. Patent Application Publication US2004/0232192 (Marks) shows a further design where the power spring is a dual, co-axial, coiled torsion spring and the lever is a vertically flat metal form. In these references the lever releasably engages an opening in the striker. The lever front end includes an upper position near the top of the housing body. The lever fulcrum is lower than this upper position end position; therefore the lever front end will move in an arcing motion rearward to pull out of the opening in the striker and disengage the striker. This action comprises the release action. At the upper position of the striker a staple on a guide track advances to be under the striker. The power spring forces the striker downward to eject the staple under the striker.
In these designs it is important that the release action occurs at a consistent position of the lever. If the release is too early the striker will not raise high enough to allow the staple to advance. If the release is too late it may not occur at all, the striker will reach its upper limit before the lever moves rearward out of engagement with the striker. To provide a reliable release point the lever fulcrum should be well below the upper most position of the lever front end. The resulting geometry provides a relatively large rearward motion of the lever at the release point with respect to the upward motion. With a large rearward motion the design will not be overly sensitive to manufacturing variations; the release occurs within a small vertical range of motion of the striker.
In a vertically compact design the power spring and lever must be as near as possible to each other vertically. Further the total vertical motion of the striker will be limited. When the lever and spring share a fulcrum the spring is under the fulcrum since the spring presses upward. In the Marks '728 patent, the fulcrum is a round post. A flat power spring presses the post tangentially under the post. The lever pivots around the center of this post. The lever pivot location is therefore spaced above the spring by the radius of the post.